Computed tomography has long been used to increases conspicuity of low-contrast objects in medical imaging, by reducing the effect of overlapping structures. Portal imaging of trucks and containers for homeland security could potentially benefit from a similar ability.
For medical applications, computed tomography is generally achieved through reconstruction of a data set containing projections obtained at angles on all sides of a container or body of interest. The ability to entirely encircle a body of interest is not always possible in the clinic, and can be challenging in certain portal inspection applications (e.g., trucks).
Two-dimensional x-ray imaging systems have been implemented for inspection of trucks and baggage, sometimes with pulsed x-ray sources. Likewise, transmission x-ray imaging is widely used for medical, security, and commercial applications in the form of two-dimensional (2D) projection (conventional X-ray) or three-dimensional (3D) Computed Tomography (CT). In CT scanning, digital image reconstruction is used to generate a three-dimensional image of the inside of an object from a complete angular series of two-dimensional x-ray images taken around a single axis of rotation. Such information about the three-dimensional structure of an object can be valuable; however in some cases it is not easy to entirely surround an object with sources and detectors, especially if the object is in motion.
Classically, radiologists have reduced the influence of overlapping structures in a three-dimensional object by counter-rotating the x-ray source and x-ray detector in parallel planes (“laminography”). This results in a “smearing” of the images of objects of interest in planes other than the focal plane into a uniform background as disclosed in T. D. Moore, D. Vanderstraeten, P. M. Forssell, Three-Dimensional X-Ray Laminography as a Tool for Detection and Characterization of BGA Package Defects, IEEE Transactions on Components and Packaging Technologies, 25(2):224-229 (2002) (hereby incorporated by reference in its entirety). The smearing can be applied with analog means (i.e., with film) or via digital tomosynthesis as disclosed in H. Matsuo, A. Iwata, I. Horiba, N. Suzumura, Three-Dimensional Image Reconstruction by Digital Tomo-Synthesis Using Inverse Filtering, IEEE Transactions on Medical Imaging, 12(2):307-313 (1993) (hereby incorporated by reference in its entirety). The counter-rotating process requires sequential refocusing to examine multiple planes, and can be difficult to apply to objects in linear motion with respect to the source/detector pairs.